The following disclosure is based on German Patent Application No. DE 101 36 507.1 filed on Jul. 17, 2001, which is incorporated into this application by reference.
1. Field of the Invention
The invention relates to a method for fabricating a geometric beamsplitter, a beamsplitter that may be fabricated employing that method, and an illumination system equipped with a beamsplitter of that type for use on an optical device, in particular, on a microlithographic projection exposure system.
2. Description of the Related Art
Numerous application areas of optics require splitting a light beam into two equal, or unequal, partial beams. Beamsplitters that are configured either as a geometric beamsplitter or a physical beamsplitter are employed for that purpose. While physical beamsplitters leave the cross-sectional areas of light beams unaltered, geometric beamsplitters split their cross-sectional areas into partial beams using, for example, a reflective surface that has openings.
In many applications, an effort is made to accurately determine, for example, in the case of so-called xe2x80x9cdosimetry mirrors,xe2x80x9d such as those employed on the illumination systems of optical devices, the relation between the reflected energy and the energy transmitted through those openings.
As an example, U.S. Pat. No. 6,028,660 discloses an illumination system equipped with a dosimetry device for use on a microlithographic projection exposure system for fabricating semiconductor devices and other types of microdevices. Systems of that type, which may be configured as wafer scanners or wafer steppers, are used for projecting patterns on photomasks (reticles) or ruled plates onto an object, for example, a semiconductor wafer, coated with a light-sensitive film, at high spatial resolutions. In order to obtain a well-defined pattern on the substrate, the substrate should be irradiated with an accurately determined quantity of energy, or dose. It is thus essential to determine the actual irradiation dose in order to allow accurately setting the necessary dose whenever necessary using a feedback device that controls the luminous output of the light source. Between its light source and its light exit, the illumination system has an output-coupling device for coupling out a fraction of the luminous energy from its light source toward an energy sensor whose energy measurements may be employed for controlling its light source, where it is essential to know the absolute transmittance of the beamsplitter in order to allow drawing conclusions regarding the absolute intensity of the light source based on the energy sensor""s energy measurements, which is necessary in the case of, for example, wafer steppers, in order to be able to accurately set the open periods of their shutters or the durations of the illumination periods for exposures. In the case of wafer scanners, it is essential that illumination doses be uniform over any given illuminated field.
There is thus need for beamsplitters that have accurately defined transmittances. These should be simple and inexpensive to fabricate.
One object of the invention is to provide a method for fabricating a geometric beamsplitter that will allow inexpensively fabricating geometric beamsplitters having accurately defined transmittances. It is another object of the invention to provide a geometric beamsplitter having an accurately definable transmittance that may be inexpensively fabricated in large numbers.
As a solution to these and other objects, the invention, according to one formulation, provides a method for fabricating a geometric beamsplitter comprising: coating a surface of a substrate consisting of a transparent material with a reflective coating that contains at least one metallic layer; creating a pattern of holes comprising a large number of transparent holes in the reflective coating; wherein the holes of the pattern of holes are created by laser processing.
In the case of the method according to the invention, a surface of a substrate consisting of a transparent material is initially coated with a reflective coating that contains at least one metallic layer whose thickness is preferably chosen such that it is substantially opaque to the light to be employed. A pattern of holes having a large number of transparent holes whose number, size, and/or distribution largely determines the beamsplitter""s transmittance is then created in that reflective coating. In accordance with the invention, those holes are created using laser processing, whereby the reflective coating is simultaneously or repeatedly irradiated at numerous locations with laser light having a suitable diameter and a suitable wavelength, energy, and duration such that transparent holes in the reflective coating arise, without destroying the substrate or areas of the reflective coating bordering on irradiated sections. The total transmittance is set mainly by varying the total number of holes and their average area.
The method is suitable for use on all types of reflective coatings that have absorptions at the laser wavelength employed for creating the pattern of holes that are sufficiently high to allow removing the reflective coating, largely via evaporation. As used here, the term xe2x80x9cmetallic layerxe2x80x9d stands for, in general, layers of materials that absorb the light employed strongly enough to allow their evaporation. Such materials are not necessarily metallic. Compared to wet-chemical etching methods, under which patterns of holes may be created employing a photolithographic process involving coating, exposure, and subsequent etching, which may also be employed, dry laser creation of patterns of holes has the advantage that it causes no chemical changes and leaves no contaminants on the reflective surface. The method, which works both in a vacuum and in air, thus imposes no special requirements on the working environment and allows inexpensively, rapidly, fabricating geometric beamsplitters having well-defined transmittances.
A pulsed laser is preferably employed for creating the holes. It has proven beneficial to employ a first laser pulse, and at least a second laser pulse, for irradiating a given area, where the laser energy is set such that the first laser pulse largely creates a hole in the coating and the second cleans up the hole in order to smooth and even out its edges. It has been found that it will be sufficient if the laser pulses have substantially the same power density and duration, which means that no special requirements are imposed on controlling the laser during laser processing and that the method may be performed less expensively than, for example, known methods for creating blind and through holes in multilayer structures for multichip modules (WO 97/44155).
It is possible to create a regular hole pattern in the form of, for example, a two-dimensional grid. However, another preferred embodiment foresees that a pattern of holes having a random distribution of holes, i.e., a pattern other than a grid, where the spatial coordinates of individual holes are generated by a random number generator, is created. All holes will preferably be randomly distributed. However, both a group of randomly distributed holes and a group of regularly distributed holes may also be provided.
Random hole distributions yield significant benefits, both during fabrication and when beamsplitters are employed. When beamsplitters are employed, i.e., when large areas of their reflective surfaces are illuminated, diffractive effects and interference effects in the transmitted light, such as those that occur in the case of patterns of transmitting holes having regular grid layouts, are precluded. Moreover, random patterns of holes may be particularly simply reprocessed to yield other random patterns of holes having greater total transmittances by creating additional (randomly distributed) holes. In the following, we shall explain how random patterns of holes may be employed for fabricating beamsplitters having accurately prescribed transmittances with high yields.
According to one embodiment of the method, in fabricating a beamsplitter having a prescribed nominal transmittance, a distribution of holes that will intentionally yield a transmittance that is less, for example, 10%-20% less, than the beamsplitter""s nominal transmittance, is initially created under controlled conditions. Although the initial hole distribution created during this initial step may be regular, it should preferably be a random distribution of holes. A measurement of the beamsplitter""s transmittance is then conducted in order to determine its current transmittance. Additional, preferably irregularly or randomly distributed, holes are then created, controlled by its nominal transmittance, in order to increase its transmittance to the desired nominal transmittance, i.e., a multistage process in which its transmittance is increased in steps until it reaches the desired nominal value, is conducted, where the steps involved in measuring its current transmittance and then creating additional holes may, if necessary, be repeated in order to gradually increase its actual transmittance to its nominal transmittance. This approach allows attaining high yields of xe2x80x9cgoodxe2x80x9d beamsplitters, since accidental fabrication of beamsplitters having excessive transmittances is reliably avoided.
Although additional holes may be created employing the trial-and-error method, the number of additional holes remaining to be created is preferably computed, based on the difference between the beamsplitter""s nominal transmittance and its current transmittance, in order to allow adding a precomputed number of additional holes that will allow reaching the nominal transmittance in a single step with high accuracy. In one embodiment of the method, an in-situ measurement of transmittance is conducted while holes are being created in the reflective surface. The creation of additional holes may then be based on the results of that measurement, which will allow confining the accuracy with which transmittance may be set to the contribution to transmittance of a single hole.
Holes created in accordance with the invention preferably have an average diameter ranging from about 30 xcexcm to about 100 xcexcm, in particular, ranging from about 40 xcexcm to about 60 xcexcm, and average interhole spacings ranging from about 0.5 mm to about 1 mm. Particularly in the case of beamsplitters destined for the preferred use on the illumination systems of microlithographic projection systems, low transmittances of less than about 5% are preferable in order that most of the light available may be employed for achieving high throughputs of exposed substrates. In practice, transmittances should be less than 3%; in particular, they should be around 1%. Accuracies of better than 30% of nominal absolute transmittances may be regularly attained.
Any suitable metallic material, either a pure metal or an alloy, may be employed for fabricating the at least one metallic layer, provided that its reflectance for the electromagnetic radiation to be employed in actual operation is sufficiently high. Aluminum is preferred in the case of the preferred application of dosimetry mirrors on illumination systems for use on microlithographic systems operating in the ultraviolet, since its reflectance over the relevant angles of incidence, which range from about 30xc2x0 to about 60xc2x0, is largely independent of the angle at which radiation is incident on it. Although the thicknesses of this layer may, for example, range from about 50 nm to about 100 nm, they are not confined to that range.
Good high-reflectance coatings based on aluminum may be evaporatively deposited at low ambient pressures and high evaporation rates employing, for example, flash evaporation. However, reproducibly providing ideal processing conditions during production requires a lot of expensive equipment and effort. A preferred embodiment of a method for fabricating a reflective layer in accordance with the invention foresees that the material of the metallic layer, i.e., in particular, aluminum, is evaporated at a gradually increasing rate, where relatively low evaporation rates are employed when evaporation commences. The evaporation rate is then gradually, preferably linearly, increased to high levels. This approach allows depositing high-quality reflective coatings, particularly in the case of aluminum, without need for a lot of expensive equipment and expenditure of a lot of effort. Under this embodiment of the method, the material to be evaporated, in particular, aluminum, is initially employed as a sort of getter pump in order to arrive at a low pressure in the coating system""s coating chamber. The low evaporation rates initially employed should be set to suit this employment. Largely uncontaminated aluminum, which forms the reflective surface, is then evaporated at high rates in this purified atmosphere. This embodiment involving an increasing evaporation rate may be beneficially employed in depositing all types of reflective coatings, even when no patterns of holes are to be created in them.
Although the reflective coating may be confined to a single metallic layer, an overcoating having at least one layer of dielectric material should preferably deposited onto the metallic layer. Although this overcoating may largely have a protective function, it is preferably designed to be a reflectance-enhancing overcoating. The metallic layer should preferably be overcoated with a multilayer coating having several alternating layers of high/low-refractive-index materials. Reflective coatings of this type are known from, for example, European Patent 0 280 299, which corresponds to U.S. Pat. No. 4,856,019, U.S. Pat. No. 4,714,308, European Patent 0 939 467, or U.S. Pat. No. 5,850,309. Any of the multilayer coatings described therein are, in principle, suitable for fabricating beamsplitters in accordance with the invention. Their associated characteristics are thus herewith made a part of this description by way of reference thereto.
Preferred multilayer coatings for use as reflective coatings on beamsplitters in accordance with the invention will be described in greater detail in conjunction with the discussion of sample applications. Of course, they may also be employed for fabricating high-reflectance mirrors for ultraviolet light having wavelengths ranging from about 193 nm to about 365 nm, regardless of whether they have had any patterns of holes applied to them.
The aforementioned and other characteristics of the invention are as stated in the accompanying claims, description, and figures, where those individual characteristics thereof depicted may represent themselves alone or several such in the form of combinations of subsets thereof that appear in an embodiment of the invention and have been implemented in other fields, as well as beneficial embodiments that may be themselves inherently patentable.